Popular Science Monthly/Volume 49/October 1896/Acetylene, The New Illuminant
|ACETYLENE, THE NEW ILLUMINANT.|
THE advent of the electric light, the Siemens-Lungren regenerative burner, and the Welsbach incandescent mantle, all within a comparatively short period, threw the lighting industry into a very unsettled condition. There had begun, however, to appear some order out of the chaos. As the special advantages of the different systems were recognized, the purposes to which each was best adapted were noted. The development of the industries was going on very satisfactorily, when a new competitor appeared in the shape of acetylene. It is now stated that Mr. Edison and Nikola Tesla are independently working out still another system, based on the vacuum-tube phenomena; a subject in which Mr. D. McFarlan Moore claims also to have made a great step in advance by the invention of his vacuum vibrator. Vacuum-tube lighting, however, is still in the laboratory, and, while surprising tales are told of its great beauty and high efficiency, it is too soon to even prophesy intelligently regarding it. Acetylene has, however, during the last few years been much discussed, and considerable data are available regarding it; so that an inquiry into its history and value as a practical illuminant is of interest.
Acetylene (C2H2) was first described by Edmund Davy, who obtained it accidentally by the action of water on a mass of carbonized tartar and charcoal powder, with which he had attempted to prepare potassium. He called the new gas Klumene. Some years later it was rediscovered by Berthelot, who obtained it by passing ethylene through a red-hot tube; he noted its occurrence in coal gas, and later succeeded in making it by passing a powerful electric current between two carbon poles in an atmosphere of hydrogen.
The resulting mixture of acetylene and hydrogen was passed into an ammoniacal solution of cuprous chloride, and the insoluble copper compound thus obtained, which is extremely explosive and has recently caused several serious accidents, was then treated with hydrochloric acid, which liberated acetylene.
Acetylene is a colorless gas, having a rather disagreeable odor, somewhat resembling garlic and phosphorus. The peculiar odor noticed when the burners of a gas stove strike back is due to the formation of acetylene. Its specific gravity when compared with air is 0·91 (ordinary coal gas has a specific gravity of about 0·607). At 0°, and under a pressure of 21·5 atmospheres (322·5 pounds per square inch), it becomes a mobile, highly refractive liquid. Water at 18° dissolves its own volume of the gas. When ignited at an ordinary burner it gives a smoky, dull flame, and with oxygen forms an explosive mixture, as it does also when mixed in certain proportions with air; but when burned through a very small tip and under slight pressure, or when mixed with coal gas or air in proper proportions, an exceedingly brilliant and highly luminous flame is produced. Acetylene is poisonous, combining with the hæmoglobin of the blood, to the exclusion of oxygen, to form a compound similar to that yielded by carbon monoxide. Moissan, however, reports that when prepared from pure calcium carbide, and after being purified by liquefaction, it has an ethereal odor, and can be breathed in small quantities without evil effects. Regarding its explosiveness Mr. J. M. Crafts says: "Experiments, using a two-inch gas pipe as a cannon, show that from five to six per cent of acetylene mixed with air forms an explosive mixture. . . . About ten per cent of water gas is necessary before an explosive mixture with air is formed." Explosive mixtures in the air of a room would be produced by much smaller percentages than these. Lechattelier gives 2·8 per cent acetylene mixed with air as an explosive compound. So far as its poisonous qualities are concerned, acetylene seems to have a little the advantage of water gas; the poisonous principle of the latter is carbon monoxide, whose combination with the blood is somewhat more energetic than that of acetylene, and which has no odor to serve as a warning in case of a leak, as has acetylene. Acetylene is, however, more prone to form explosive mixtures. This is due to the fact that in the combination of carbon and hydrogen to form acetylene 61·100 units of heat are absorbed. Thus the heating power of a cubic foot of acetylene is sufficient to raise 407 kilogrammes (a kilogramme = 2·2 pounds) of water 1° C. The combustion of the same amounts of uncombined carbon and hydrogen as are present in a cubic foot of acetylene will raise only 336·5 kilogrammes of water to 1° C, leaving a difference in favor of acetylene of 70·5 heat units—the unit being the amount of heat required to raise the temperature of one kilogramme of water 1° C.
Acetylene is one of the important bodies, much used by the chemist in the synthesis of organic compounds. It is also reported to be of value in polariscope work, permitting the reading of solutions so highly colored as to be opaque to the ordinary sources of light. Some interesting experiments with the gas in abnormal physical states were recently performed by J. J. Suckert, during a lecture before the Franklin Institute. A tube of liquefied acetylene was cooled to-28° F., and then the pressure removed. Rapid evaporation took place, and a portion of the liquid gas was solidified into a snowlike mass whose temperature was found to be-118° F. A part of this snow placed on some mercury in a saucer soon froze the latter, and another portion, on being dropped into water, upon which it floated, gave off acetylene gas, which was readily ignited and burned with its characteristic sooty flame.
Regarding the illuminating power of acetylene, a proper burner using five cubic feet per hour will give from two hundred to two hundred and forty candle power. Five cubic feet of ordinary gas give from fifteen to thirty candle power; that is, a cubic foot of acetylene will give about eight times as much light as the same amount of coal or water gas. Indeed, it is claimed by Prof. Lewes and others that the formation of acetylene in the ordinary gas flame accounts for the latter's luminosity, and it has been proposed to enrich water gas by the addition of a small amount of acetylene; but so much of the latter was found necessary to produce any appreciable result as to render the process impracticable. Acetylene requires a much larger amount of air for complete combustion than does ordinary gas. This is a distinct disadvantage, as the large amount of air cools the flame, and thus diminishes its luminosity. The temperature of the acetylene flame is about 1000° C, that of an ordinary flat coal-gas flame being 1360° C.
The present rise into prominence of acetylene, which up to 1888 was simply a laboratory product, is due to the discovery of the formation of calcium carbide in the electric furnace. There is some controversy as to who first made this discovery, but the honors seem to belong to Mr. T. L. Wilson, of the Wilson Aluminum Works. In 1888 Mr. Wilson began a series of experiments with the electric furnace for reducing refractory ores; during one of these a curious, dark-brown, dense mass was formed, whose immersion in water produced a violent evolution of gas, which upon investigation proved to be acetylene. A French chemist, Moissan, independently discovered the process, and reported it at the meeting of the French Academy, in December, 1892. But as Mr. Wilson sent samples of the carbide to Lord Kelvin in the summer of 1892, for examination, he seems to have preceded Moissan, at any rate in announcing his discovery. All the alkaline earths form carbides in a similar way, which, when treated with water, give off acetylene. It may be interesting to note, in passing, that by means of the electric furnace, a carbide of silicon has recently been obtained, which under the name of carborundum is coming to be used extensively as a polishing and grinding material. It is extremely hard (scratching rubies) and is said to wear well. Another interesting product is the carbide of titanium, the hardness of which is sufficient to scratch the diamond. This discovery of the ready formation of carbides under the great heat of the electric furnace is of special interest to the geologist, as bearing on the theory that these carbides are present in large quantities in the interior regions, to which water must occasionally penetrate; the resulting generation of gases and production of high pressures and heat accounting for the various volcanic disturbances and the large natural deposits of petroleum and other carbonaceous material, which occur so abundantly in some districts.
Pure calcium carbide has a specific gravity of 2·262; in a dry atmosphere it is odorless, but upon exposure to moisture evolves the peculiar odor of acetylene. When exposed in lumps to the action of ordinary air it becomes coated with a layer of hydrate of lime, which protects the interior of the mass from further oxidation. It is not inflammable, and can be exposed to the heat of the ordinary blast furnace without decomposition. It is, in fact, a very stable compound, its ready decomposition under the action of water being quite at variance with its other chemical properties. It was first prepared by Woehler, in 1862, by fusing an alloy of zinc and calcium with carbon. He called it acetylene carbide. It forms a dark grayish or red dense mass, which upon fracture shows a crystalline metallic surface. The whole process of manufacturing acetylene, from the preparation of the lime and coke onward, is very simple, and the only reason why it is new as a commercial product is the difficulty of causing a combination between the calcium of the lime and the carbon of the coke. Nothing short of the temperature of the electric furnace (3500° to 4000° C.) will bring this about, and the comparative modernness of this apparatus accounts for the lateness of the calcium carbide. The chemistry of the process is as follows: Quicklime (CaO) and coke, or any other substance whose main content is carbon (C), are mixed and fused together in the electric furnace. The calcium (Ca) of the lime combines with part of the carbon (C) of the coke to form calcium carbide (CaC2); the oxygen (O) of the quicklime combining with another portion of the carbon to form carbonic oxide:
Carbonic oxide is a gas and is driven off, leaving calcium carbide and the various impurities in the furnace. The further reaction to form acetylene occurs when calcium carbide is subjected to the action of water:
The following description of the commercial manufacture of calcium carbide as conducted at Spray is based on a paper by G. de Chalmot, who for some time had personal supervision of the works of the Wilson Aluminum Company at Spray, N. C, and an address by W. R. Addicks, of Boston, Mass., delivered before the New England Association of Gas Engineers at their twenty-sixth annual meeting:
The electric furnace is of ordinary brick, two and a half by three feet (inside measurements) at the bottom. The front side consists of four iron doors. The electric current enters at the bottom and top; the lower electrode is an iron plate covered with eight inches of carbon (pieces of carbon pencils or a mixture of coke and coal tar). Sixteen copper cables 0·75 inch in diameter convey the electricity from the dynamos to the bottom electrode; sixteen other cables are connected with the top electrode, which is composed of six carbon pencils each four inches square and thirty-six inches long; these are bound together by a sheet of iron, so as to really form only one pencil. The upper electrode is so arranged that it can be raised or lowered by means of a screw. Dynamos giving a current of from fifty to one hundred volts and seventeen hundred to two thousand ampères are used, actuated by a water wheel of three hundred horse-power capacity. To start the furnace a little carefully ground and mixed lime and coke (this being done by special grinding and mixing machinery, which forms an essential part of the plant) is thrown on the bottom of the furnace, the current turned on, and the upper electrode lowered until an arc is formed between it and the mixture. The carbide soon begins to form, and new material is shoveled in as the ingot is built up. The end of the pencil is kept covered with about a foot of the mixture, and is gradually raised by the attendant until the capacity of the furnace is reached; then the current is turned off and the furnace left to cool. This constitutes the whole process, and is extremely simple and inexpensive, requiring no skilled labor and little machinery. Much time has been lost at Spray in waiting for the furnace to cool, which requires from four to eight hours. In the new plant of the Philadelphia company at Niagara the lower electrode and the bottom of the furnace consist of a car, which, as soon as the run is finished, can be drawn out and a fresh car substituted, thus obviating the loss of time and heat in waiting for the furnace to cool. Many other improvements, including an arrangement by which the mixed lime and coke are automatically fed into the furnace, are expected to materially reduce the cost of manufacture at the Niagara works.
The proportions of lime and coke are roughly calculated by means of the atomic weights involved in the reaction, but in practice it is found that, owing to impurities and loss in the process, these amounts have to be exceeded somewhat. After the mass in the furnace has cooled sufficiently, it is dumped on a grate which holds the carbide and permits the unreduced material, amounting to from fifty to seventy-five per cent of the original mass, to fall through into a bin, from which it is collected to be used again. The lime requires to be fairly pure, over five per cent of impurities interfering seriously with the production of carbide. Magnesia is very undesirable, and it is stated that if over three per cent is present a good quality of carbide can not be made. This matter of impurities and the care of the carbon pencils, which when properly looked after wear away only about 0·09 of an inch per hour, but which may make a great deal of trouble if carelessly tended, are the points requiring special attention. Unslaked lime is said to give the best results. The alternating current is used at Spray, but a direct current can be employed. Besides coke, soft coal, anthracite, charcoal, pitch, tar, resin, and asphalt have been tried in combination with lime. Indeed, the first mixture used by Mr. Wilson was lime and tar, which had been boiled together in a caldron and then thoroughly dried. With the exception of charcoal, however, none of these substances were found of any value as compared with coke, although they all produced some carbide. As regards the amount and quality of the light obtained from acetylene properly burned, there seems no question as to its great superiority over either ordinary coal or water gas. It stands about even with water gas in poisonous qualities, but is more liable to explosion. Greater care would be required in handling it, especially if the proposition, which was at first well received, to use it in a liquefied state from cylinders under great pressure, should prove practicable.
Its success as an illuminant, however, depends almost entirely on the cost of manufacture, and regarding this point it is somewhat difficult at present to get reliable data, chiefly, perhaps, because of the experimental stage in which much of the apparatus still is.
The Progressive Age, a New York publication devoted to the interests of electricity, gas, and water, recently formed a commission which it sent down to Spray for the purpose of determining the actual cost of manufacturing calcium carbide. The commissioners were Prof. Houston and Drs. Kennelly and Kinnicutt, and their conclusions, after careful examination of the works and a testing of two full runs, were published in the Progressive Age for April 15th, and are as follows:
"Our estimate, therefore, of the cost of producing calcium carbide at Spray, by working the furnaces three hundred and sixty-five days a year and twenty-four hours a day, yielding on the average one ton of two thousand pounds gross carbide a day, is $32.76 per ton. Of this amount $14.39 is for material. The freight charges on lime and coke are heavy at Spray, and add materially to the cost."
They found an average net yield of acetylene of 4·926 cubic feet of moist gas per pound of net carbide, or 4·696 cubic feet per pound of gross carbide. A gross ton of carbide, then, will yield about 9,400 cubic feet of acetylene at a cost very slightly above $32.76, as the final formation of acetylene is practically without expense. Nine thousand four hundred cubic feet of coal or water gas costs the consumer somewhere between fourteen and sixteen dollars, roughly about one half as much as the same amount of acetylene. But as acetylene gives over eight times as much light per cubic foot, a large margin seems to be left for decreased gas bills to the consumer and increased profit for the producer. On the whole, then, it may be said that acetylene promises to be an important rival of the present methods of illumination, and deserves the careful examination of both the consumer and manufacturer of light-givers.
It has been proposed to use calcium carbide as a concentrated fuel on war vessels and in places where space is of more importance than cost. Dr. Frank has made the following calculation of the gain in space over coal: To provide power for a one-thousand-horse marine engine for twenty-five days would require four hundred and thirteen tons of coal, occupying a space of about fifteen hundred cubic feet. In this space could be stored enough of the carbide to propel the ship at the same rate for seventy-five days. In other words, as a fuel one ton of carbide is equal to three tons of coal. There are works engaged in the manufacture of calcium carbide at Spray, N. C, at Niagara Falls and at Lockport, N. Y.; in Europe, at Betterfeld, Prussia, at Neuhausen, Switzerland, at Baden, in Germany, and at Troyes and Vallorbes, in France.